As for internal combustion engines mounted in automobiles, it is important to reduce NOx (nitrogen oxides) and PM (particulate matter) contained in the exhaust gas. One of the methods for purifying the exhaust gas in order to reduce the NOx is a method for exhaust gas purification in which an ammonia solution, such as urea, is injected into an exhaust pipe so as to generate ammonia by hydrolysis, and a NOx catalyst of the selective reduction type (SCR catalyst) selectively reducing NOx on an adsorption material, such as zeolite.
This SCR (Selective Catalytic Reduction) system is widely used as an aftertreatment device for exhaust gas from diesel engines. In this SCR system for diesel engines, NOx has to be reduced in an oxidative atmosphere. For this reason, a reducing agent, such as aqueous solutions of ammonia (NH3) and urea (NH2)2CO (urea water), is supplied to the exhaust gas, and then NOx is decomposed into nitrogen (N2) and water (H2) by a NOx catalyst of the selective reduction type which has selectively adsorbed NOx in the exhaust gas, thereby being discharged.
When urea or an aqueous solution of urea is supplied, urea is hydrolyzed by heat of the exhaust gas in the reaction: (NH2)2CO+H2O→2NH3+CO2 to generate NH3. Then, in the NOx catalyst of the selective reduction type, NOx is reduced to be purified mainly in the reaction: NO+NO2+2NH3→2N2+3H2O. It should be noted that an oxidation catalyst may be disposed in some cases in the downstream side so that surplus NH3 flowing out to the downstream side of the NOx catalyst of the selective reduction type can be oxidized to be removed in the reaction: 4NH3+3O2→2N2+6H2O.
In the meantime, as one of the methods for reducing PM, a DPF (diesel particulate filter) is available which temporarily traps PM with a filter made of ceramic or the like so as to reduce PM while regenerating the filter by burning thus trapped PM for removal. In this DPF, HCs (hydrocarbons), such as fuels, are supplied to be burned. Thus generated combustion heat is used to raise the temperature of the DPF so that the trapped PM can be burned to be removed. It should be noted that catalyst-supported DPFs are also widely used in which a precious metal catalyst is supported on a filter so that a combustion initiation temperature of PM can be lowered.
In the prior art, post injection has been the most common method for supplying HCs, the post injection performing fuel injection during the expansion stroke in the combustion cycle inside a cylinder. However, since injection is performed in this post injection when the temperature inside the cylinder drops, a part of the fuel falls from the cylinder liner into oil. This brings about a problem of oil dilution, in which lubricating oil is diluted with the fuel. For this reason, in recent years, it is more preferred that HCs, such as fuels, be directly injected into an exhaust pipe.
However, the following problem is involved in a system for exhaust gas purification employing both a catalyst of the selective reduction type and a DPF. In general, when a NOx catalyst of the selective reduction type is used, an oxidation catalyst (DOC) is disposed in the upstream of the NOx catalyst of the selective reduction type. This is because the optimal condition for the reduction reaction of NOx can be achieved when the ratio between NO (nitrogen monoxide) and NO2 (nitrogen dioxide) is NO: NO2=50:50, and therefore, a condition close to this condition is to be achieved by oxidizing NO to NO2 with the oxidation catalyst.
In the meantime, when injection is performed with an injection valve for injecting an ammonia solution disposed in the upstream side of the oxidation catalyst, NO2 is generated from NH3 (ammonia) by the oxidation catalyst. For this reason, this injection valve needs to be disposed downstream of the oxidation catalyst and upstream of the NOx catalyst of the selective reduction type. Further, mixing of the exhaust gas and ammonia largely affects the purification rate of NOx in a NOx catalyst of the selective reduction type. Accordingly, it is generally necessary to provide a mixer in the upstream side of the NOx catalyst of the selective reduction type for the purpose of increasing the mixing effect.
On the other hand, a fuel injection valve for direct injection into an exhaust pipe for the purpose of regeneration of DPF needs to be disposed in the upstream side of the oxidation catalyst so that HCs can be oxidized by the oxidation catalyst to generate heat. In order to achieve this, in arrangement in the prior art, as shown in FIG. 4, a case 10X for an oxidation catalyst (DOC) 17 and a catalyst-supported DPF (CSF) 14 and a case 10Y for a NOx catalyst of the selective reduction type (SCR) 12 are formed independently from each other, and a fuel injection valve 21 and an injection valve 20 for an ammonia solution are provided in the upstream sides of the cases 10Y and 12, respectively.
However, in such a system 1x for exhaust gas purification, since a mixer 27 is disposed after the injection valve 21 for an ammonia solution, the system becomes large scale. Therefore, there arise problems that arrangement (layout) in installation of this system into automobiles and the like is difficult, and that costs for the system are high.
As one of the countermeasures taken for this, for example, as described in Japanese patent application Kokai publication No. 2006-266192, an apparatus for purifying the exhaust gas of engines is proposed, which has a relatively simple structure in which a second oxidation catalyst, a liquid injection nozzle for supplying an aqueous solution of urea, a catalyst of the selective reduction type (SCR catalyst), a first oxidation catalyst, and a DPF are provided in order from the upstream side of an exhaust passage; and the catalyst of the selective reduction type, the first oxidation catalyst and the DPF are housed in a single converter.
In this apparatus for purifying the exhaust gas of engines, while the NOx purification efficiency on the catalyst of the selective reduction type is improved by oxidizing NO in the exhaust gas to NO2 with the second oxidation catalyst, the amount of hydrocarbons in the exhaust gas is increased and decreased by post injection, and the hydrocarbons are oxidized by the catalyst of the selective reduction type and the first oxidation catalyst so as to generate heat. Thereby, PM trapped by the DPF is burned to be removed.
However, in this apparatus for purifying the exhaust gas of engines, since HCs, such as fuels, are injected into the exhaust pipe not by direct injection but by post injection, a part of the fuel at the time of the post injection falls from the cylinder liner to oil. This brings about a problem of oil dilution, in which the lubricating oil is diluted with the fuel. In addition, there is a problem that, by using post injection, this apparatus would not be a useful reference for making a system for exhaust gas purification more compact in the case of achieving both urea supply and HC supply by direct injection of HCs into an exhaust pipe.